1. Field
The present invention relates generally to the preparation of high activity supported metal catalysts and specifically to a method for producing a high activity metal catalyst supported on a porous support material using a supercritical solvent to promote maximum adsorption of the metal on the support.
2. State of the Art
Catalysts are required in a large number of important industrial and commercial chemical reactions to insure that these reactions will proceed at moderate temperature and pressure conditions to produce a high yield of the desired product. Exemplary of such processes are the various hydrogenation reactions that accompany petroleum cracking, the oxidation of olefinic hydrocarbons and the oxyacylation of gas phase olefins. These and many other important industrial chemical reactions proceed very slowly or not at all at ambient temperatures and pressures unless a suitable catalyst is added to the reactants. Consequently, a great deal of effort has been expended both to develop new catalysts that will function as required to promote such reactions and to improve existing catalysts. An ideal industrial catalyst must be economical to manufacture and must possess reproducibly high catalytic activity. Achieving both objectives. however, has proven somewhat elusive, and the patent art chronicling these efforts is voluminous.
Of the many types of available catalysts, one particular class of catalysts has received considerable attention because of its role in catalyzing such important industrial reactions as hydrogenation and methanation. This type of catalyst includes the supported metallic catalysts, which are formed primarily from transition metals such as, for example, nickel. iron, cobalt, or precious metals such as platinum, rhodium, palladium and silver. Although these metals may be used alone or in porous geometries to catalyze reactions, they are more economically employed in combination with a high surface area porous support material. A high specific area film of the metal catalyst may be deposited on the surface of a porous support material, usually an oxide of such elements as aluminum, zirconium, beryllium or magnesium. Since it has been discovered that many of these metal oxide support materials also possess some catalytic activity themselves, the total catalytic activity of the metal film--metal oxide support may be greater than that available from the pure metal alone. U.S. Pat. Nos. 2,773,844; 4,093,559; and 4,142,962 are exemplary of available prior art methods for making supported metal catalysts.
One method of producing a high activity supported metal catalyst includes casting a base of the support material. e.g., alumina or zirconia, which is typically a porous body to achieve maximum surface area. This porous support body is first evacuated to remove gaseous surface contamination and then immersed in an aqueous solution of a soluble salt of the metal selected to function as the catalyst. The aqueous metal solution penetrates the porous support body, and the metal salt is adsorbed on those surfaces of the support which it is able to contact. The metal-ceramic composite is then dried and. optionally, may be calcined to convert the metal salt to a metal oxide. Any free ionic species deposited with the metal salt must be driven off to avoid the formation of a nonvolatile residue, such as chloride or sulfate, which might poison the catalytic activity of the final composite. Activation of the catalyst is accomplished by heating the metal-ceramic composite in a reducing atmosphere, typically hydrogen. This converts the metal salt surface film on the ceramic support to the pure metallic form so that it is available to function as a catalyst.
One of the problems associated with this process, however, arises from the extremely small diameter of the pores of the ceramic support material. Alumina, for example, may have an effective pore diameter of only 0.1-1.0 microns. An aqueous solution of a metal salt encounters difficulty penetrating completely fluid passages of such small size because the surface tension and viscosity of the solution tend to impede its flow. While evacuation of the ceramic base assists penetration of the aqueous metal solution into the pores of the base material, optimum wetting of the surfaces by the metal salt solution does not occur. Since only those surfaces wetted by the metal impregnation solution will ultimately receive a metallic film, much of the potential catalytic activity of the composite will be lost if the metal solution does not completely penetrate the ceramic pores. Therefore, achieving maximum penetration of the pores of the support by the metal solution so that the maximum surface area will be contacted by the solution, thereby allowing the metal to be adsorbed on the support, is critical to obtaining maximum catalytic activity.
It has been proposed to improve impregnation of the metal into the pores of the ceramic support material by the use of a gas phase impregnant, such as nickel carbonyl. However, the toxicity of nickel carbonyl poses other processing difficulties.
U.S. Pat. No. 3.518,207 discloses a process whereby particles of alumina are contacted with carbon dioxide-saturated, platinum-containing solution to prepare a platinum-alumina reforming catalyst. Gaseous carbon dioxide is continuously fed to the platinum solution to maintain saturation, and the solution thus produced is continuously recirculated through a bed of alumina particles for a period of about an hour, apparently at ambient temperatures and pressures. The method described in this patent is likely to achieve some improvement in pore penetration over that achieved by an aqueous solution containing the metal catalyst alone. However, the surface tension and viscosity of the platinum-containing solution will not be significantly changed by saturating the solution with carbon dioxide under the conditions disclosed in the patent, and maximum contact of the alumina surfaces is not likely to be achieved.
An improvement in penetration of a support material by a metal catalyst is disclosed to be achieved by the method of U.S. Pat. No. 2,696,475. According to this method, a porous carrier is soaked in a solution containing the metal catalyst in complex ion form, the excess solution is removed, and the metal catalyst-carrier material is treated with a gas containing carbon dioxide to form a metal carbonate in situ on the carrier. It is suggested in this patent that the treatment with the carbon dioxide-containing gas may be conducted at super-atmospheric pressures to speed the carbonation reaction and assure complete penetration. However, because this method initially deposits the metal on the support in the form of an aqueous solution, the viscosity and surface tension limitations of aqueous solutions will initially limit the penetration of the metal into the carrier pores.
U.S. Pat. No. 4,550,093 discloses a type of supported catalyst, useful as a Ziegler-Natta polymerization catalyst, wherein a porous alumina-based aerogel support having high surface area is impregnated by a transition metal compound. Successful impregnation of the aerogel by the transition metal, which is disclosed to be dissolved in a heptane solution, requires the maintenance of anhydrous conditions and is conducted at ambient temperatures. While the impregnation method described in this patent might achieve maximum catalyst metal penetration in the specific aerogel support described in this patent, it is not likely to be universally applicable to enhance metal catalyst penetration on other types of porous catalyst supports.
The prior art, therefore, has failed to disclose a method of making a high activity metal catalyst supported on a high surface area porous support which achieves maximum penetration of the pores of the support and, therefore, maximum surface area deposition by the metal catalyst.